The experiment described in this proposal is designed to assess changes in the rate and timing of information processing that are associated with age, physical fitness, and processing complexity. To characterize these relationships, relative changes in the latency of the P300 component of the event-related brain potential (ERP) and of reaction time (RT) will be analyzed. The thrust of this research is to differentiate age- from health-related changes in the timing of mental processes, to identify the stages of information processing where these changes occur, and to evaluate the contributions that chronic aerobic exercise makes to the maintenance of information processing speed in the elderly. Thus, speeded information processing will be studied in older adults who vary in levels of physical fitness and activity, and their performance will be compared to that of young active and inactive adults. Fitness levels will be determined by objective medical criteria so that age and health factors are not confounded. Stimulus discriminability and response compatibility will be manipulated orthogonally in this experiment. Subjects will be shown a matrix containing the word LEFT or RIGHT and will be required to respond with the left or right hand. The word will be embedded in a matrix of letters chosen randomly from the alphabet or in a matrix of number (#) signs. On some trials, compatible responses will be made (e.g., RIGHT signals a right hand response), while on others incompatible responses will be made (e.g., RIGHT signals a left hand response). Previous research indicates that the timing of the P300 component of the ERP is sensitive to changes in the ease with which a stimulus can be processed, but is relatively insensitive to changes in the facility with which a response can be selected. Analyses of the relative timing of P300 latency and RT will, therefore, permit inferences to be drawn about the loci of age- and health-related changes in the rate and timing of information processing, and the preservative effects of exercise on these functions.